In fabrication of microelectromechanical system (MEMS), deflectable or movable structures are typically produced by etching features into a device layer, using silicon processing techniques common to the semiconductor industry to form the structure's form. The deflectable structures are often held immobile initially by a layer of sacrificial material. Typically, the layer of sacrificial material underlies the deflectable or movable structure. The underlying sacrificial layer is subsequently removed (e.g., by preferential etching) in a release process to produce a suspended deflectable structure or, in some cases, a free element. Often the structural device layer is silicon, a silicon compound, a metal, or an alloy. Various sacrificial materials such as silicon dioxide, photoresist, polyimide, epoxy, wax, polysilicon, and amorphous silicon have been used for the sacrificial layer. Some MEMS devices are made by using two or more sacrificial materials for support, immobilization, and/or release of different structures of the MEMS device, which may have more than one structural device layer. The various sacrificial materials may be removed by the same etch process or by different selective etch processes. For example, a first sacrificial material or a portion of it may be removed by a wet etch and a second sacrificial material and/or a remaining portion of the first sacrificial material may be removed by a plasma etch.
Some specific sacrificial materials and etchants that have been used with the sacrificial materials include silicon oxide, removed, e.g., by hydrofluoric acid (HF) or buffered HF etching; amorphous silicon, removed, e.g., by xenon difluoride (XeF2) etching; and organic materials such as photoresist removed by oxygen plasma ashing.
After release by removal of the sacrificial material(s), the MEMS structures may be subject to ambient conditions which can lead to particulate and chemical contamination while the MEMS wafer is being stored, being inspected, or being prepared for packaging. Standard practice in MEMS fabrication often includes enclosing the MEMS devices within a package that protects the MEMS devices from environmental effects after MEMS release. The package may be hermetic, and the MEMS fabrication process may include bonding.
It has been reported that the greatest single cause of yield problems in fabrication of MEMS structures is “stiction,” unwanted adhesion of a MEMS structural element to another surface. Various coating materials have been employed to help prevent stiction. Such anti-stiction coatings are commonly applied after release of the MEMS device structures. Some anti-stiction coatings that have been used include amorphous hydrogenated carbon, perfluoropolyethers, perfluorodecanoic acid, polytetrafluoroethylene (PTFE), diamond-like carbon, and an alkyltrichlorosilane monolayer lubricant. Dessicants are also sometimes used in MEMS packages to help keep moisture away from device structures.
When bonding of a package seal occurs after MEMS release, packaging processes, including desiccant introduction or anti-stiction coating can lead to particulate generation and chemical contaminants on the MEMS devices.
Other steps of many packaging procedures may require processes that can also adversely affect the MEMS structures if they are in a fully released state. For example, soldering or anodic bonding can lead to thermally or electrically induced strain and/or bending in the MEMS structures. Radiation, e.g., ultraviolet (UV) radiation used for curing epoxies, has the potential to damage fragile circuits through solid-state interactions with high-energy photons and can indirectly lead to heating, causing problems as described with reference to soldering or anodic bonding. High electric fields, such as the fields that may occur in anodic bonding, can damage MEMS by causing “snap-down,” charge-trapping, and other unwanted electrical phenomena. Outgassing of organic materials, e.g., in adhesive curing, can lead to surface adsorbed contamination of sensitive MEMS areas causing corrosion, stiction, charge-trapping, or other dielectric-related phenomena. Deposition of an anti-stiction coating after MEMS release, but before plasma-assisted bonding, may lead to fouling of the bonding surfaces. Conversely, high-temperature bonding processes may adversely affect the anti-stiction coating. Thus, if the anti-stiction coating is placed in or on the MEMS device after release, but before package seal bonding, process integration problems may arise, such as surfaces that will no longer bond, or, an anti-stiction coating that loses functionality for the MEMS due to thermally induced chemical changes.
Thus, an improved MEMS fabrication method is needed to minimize or avoid these shortcomings of the prior art.